Bimetallic furnace blower controller switch



A. E. BAAK June 23, 1964 IMETTALLIC FURNACE BLOWER CONTROLLER SWITCH 2 Sheets-Sheet 1 Filed April 10, 1961 u ALBEQT 1,551:

INVENTOR.

BY #15 4rroR gs.

A. E. BAAK June 23, 1964 BIMETTALLIC FURNACE BLOWER CONTROLLER SWITCH 2 Sheets-Sheet 2 Filed April 10, 1961 fla 1900M mskmasmr Bum/E2 VALvE I I l/snree B lam/E1? RELAY .19

ALBEQT 344K,

INVENTOR.

BY HIS Arizona/e 15 ,HivQe/s KIECI-QRUSSELL KEA/l.

United States Patent 3,138,684 BIMETALLIC FURNACE BLOWER CONTROLLER SWITCH Albert E. Baak, 640 Ocampo Drive, Pacific Palisades, Calif. Filed Apr. 1t), 1961, Ser. No. 101,871 Claims. (Cl. 200-422) This invention relates to a switch for controlling the blower of a furnace or the like. Typically, a furnace will include a burner and vented combustion chamber, a heat exchanger positioned around the combustion chamber, ducts for conducting heated air from the heat exchanger to the area being heated, and a blower for moving air through the heat exchanger and the ducts. Fuel is supplied to the burner through a solenoid-operated burner valve, with the solenoid being energized from a thermostat positioned in the area being heated.

At the time the room thermostat closes, energizing the burner valve solenoid and supplying fuel to the burner, the heat exchanger is filled with cold air. Therefore it is not desirable to have the blower start at the time the burner valve is energized, and various types of delay devices have been utilized. At a later time, when the room temperature has reached the desired value and the room thermostat opens, the heat exchanger is still hot and additional heat can be extracted from the furnace. Hence it is customary to keep the blower operating for a period of time after the burner has been shut down in order to-conserve this heat. Blowers are typically turned oif by utilizing a thermal responsive element such as a bimetal strip positioned in the heat exchanger, with the element shutting off the blower when the temperature in the heat exchanger falls below a predetermined value.

An undesirable condition, usually referred to as short cycling, often occurs in furnace blower operation. When the thermal responsive element in the heat exchanger is used as the sole control for the blower, the blower is turned on when the air temperature in the heat exchanger attains a certain value and is turned off when the air temperature falls below another value, the latter value ordinarily being lower than the former value because of the operating gap of the element. When the burner shuts off, the air temperature in the heat exchanger drops rather rapidly because the blower is still operating. Then the blower will be shut off by the thermal responsive element. However, because of residual heat in the corn-v bustion chamber, after a period of time the air in the heat exchanger will be heated to a temperature suflicient to again turn on the blower. The blower will operate for only a short period and will again be shut off. This operation may occur several times before all of the residual heat is lost from the combustion chamber. This short cycling operation is undesirable because the air circulated through the ducts is relatively cooled and because of the noise produced by the blower starting and stopping.

It is an object of the present invention to provide a furnace blower switch for controlling blower starting and stopping and eliminating short cycling. Another object is to provide such a switch which may be enclosed in a single unit and mounted directly on the furnace.

It is a particular object to provide a furnace blower switch which will turn on the blower only when the burner valve is energized thereby eliminating any possibility of short cycling of the blower. A further object is to provide such a switch wherein the delay between burner valve energization and blower start may be set to a predetermined value. A further object is to provide such a switch wherein the delay may be made independ- 3,138,684 Patented June 23, 1964 ent of voltage supply variations and ambient temperature variations.

It is an object of the invention to provide a furnace blower switch in which the blower shutoff operation is controlled solely as a function of temperature of the air being circulated by the blower to provide optimum control of air temperature for customer comfort. A further object is to provide such a switch which is inexpensive to manufacture, simple to adjust and reliable in operation.

It is an object of the invention to provide a furnace blower switch including a housing adapted for mounting on the furnace wall, a blower switch mechanism carried in the housing and movable between. circuit open and closed conditions, a first thermal responsive element mounted in the housing and movable between normal and actuated positions, means for coupling the first element to the blower switch mechanism for actuating the blower switch, a latch mounted in the housing and movable between latched and unlatched positions, means for urging the latch toward the latched position to engage and retain the first element when the first element is in the actuated position, a heat source positioned at the first element for moving the first element to the actuated position, means for energizing the heat source when the furnace blower is energized, and another thermal responsive element carried on the housing and projecting into the furnace and movable between normal and actuated positions, with the other element disposed to engage the latch when moving to the normal position to move the latch to the unlatched position, release the first element and actuate the blower switch to the open condition. A further object is to include such a switch in which the means for energizing the first heat source includes a voltage compensation switch and a second heat source connected in series with the first source, with the compensation switch having an additional thermal responsive element for opening and closing the compensation switch, and with the second source positioned at the additional element for opening the compensation switch when energized above a predetermined potential.

It is another object of the invention to provide such a switch including first means energized from a voltage source and responsive to the furnace burner energization for moving the blower switch to the blower on condition at a predetermined time following energization of the blower, a latch mechanism mounted in the housing for latching the blower switch in the on condition, and second means responsive to the air temperature in the furnace for engaging the latch mechanism to unlatch the blower switch when the furnace air temperature falls below a predetermined value. A further object is to provide such a switch incorporating voltage compensationmeans coupled between the first means and the voltage source for controlling power flow to the first means as a function of the potential of the source to maintain the predetermined time substantially constant.

The invention also comprises novel details of construction and novel combinations and arrangements of parts, which will more fully appear in the course of the following description. The drawings merely show and the description merely describes a preferred embodiment of the present invention which is given by way of illustration or example.

In the drawings:

FIG. 1 is a front view of the switch mounted on a furnace wall;

FIG. 2 is a side view of the switch of FIG. 1;

FIG. 3 is an enlarged view of the back side of the switch with the mounting plate removed;

FIG. 4 is a sectional View taken along the line 4-4 of FIG. 3;

FIG. is an enlarged partial sectional view of the switch as shown in FIG. 2;

FIG. 6 is a motion diagram of the latch mechanism of FIG. 5;

FIG. 7 is an exploded isometric view of the latch mechanism; and

FIG. 8 is a schematic diagram showing the switch of the invention connected into a furnace control circuit.

Referring first to the schematic of FIG. 8, the control circuit is typically energized from an AC. source through a stepdown transformer 10 which provides a nominal twenty-four volts at the secondary winding 11. The switch of the invention 12 is enclosed in dashed lines. One branch of the control circuit includes a room thermostat 13 and a burner valve solenoid 14, with a resistance heater 15, a contact pair 16, and another resistance heater 17 in parallel with the solenoid 14. The heaters 15 and 17 are also identified as heaters A and B, respectively, in the various figures of the drawings. A second branch of the circuit includes another contact pair 18 and a blower relay 19.

Referring now to the mechanical construction of the switch, as best seen in FIGS. 1, 2 and 6, a snap action switch is contained in a molded plastic case 25. The open side of the case 25 is closed with a plate 26 and the unit is mounted on the top of a metal case 27 by screws 28. The case 27 is fixed to a mounting plate 29 by screws 30. A bracket 31 is afiixed to the opposite face of the mounting plate 29 and carries a thermal responsive element, such as a bimetal strip 32 therein.

The complete switch unit may be mounted on the exterior wall 35 of a furnace with the bracket 31 projecting into the furnace through an opening 36 in the furnace Wall. The bimetal strip 32 is thus positioned within the furnace so as to be responsive to the temperature of the air. which is circulated through the ducts by the blower.

The snap action switch in the case 25 carries the contact pair 18, which is shown in the closed condition in FIG. 5. The design and operation of this switch may be conventional and the particular switch shown in this application is illustrated and described in detail in my copending application Serial No. 845,007, filed October 7, 1959, now Patent No. 2,985,738 and entitled Switch. In this particular switch, one contact of the pair 18 is-fixed and the other is carried at the end of a moving arm 37. The switch is adjusted to be normally biased to the circuit closed condition and is moved to the circuit open condition by applying a force to the member 37, to the left as seen in FIG. 5, via a push rod 38. Alternatively, the rod 38 could be fixed to the arm 37 and function to move the arm both to the open circuit and closed circuit conditions.

The structure of the bimetal strip 32, the suspension for the strip in the bracket 31 and the blower off temperature adjustment lever 41 may be conventional. The particular structure shown in FIGS. 1, 2 and 5 corresponds to that shown in my aforesaid copending application and will not be described in detail herein.

The interior of the case 27 is divided into compartments 43, 44 by an insulating board 45. A U-shaped member 46 is mounted in the compartment 44 for pivoting movement relative to the case. The U member 46 includes a channel 47 as the bight, a thermal responsive element in the form of a bimetal strip 48 as one arm, and another thermal responsive element in the form of a bimetal strip 49 as the other arm. The channel 47 includes pivoting extensions 50 which are positioned in corresponding openings in the case 27, with a terminal board 51 of insulating material fixed to the channel between the pivoting extensions. The heater 15 is carried on the bimetal strip 48, the heater preferably comprising a plurality of turns of resistance wire wound over an insulating sheet 52.

A latching mechanism comprising a latch lever 61), a pivot arm 61 and an adjustment plate 62 is also mounted in the compartment 44. The lever 60 is mounted on a pin 63 carried in the case, with a coil spring 64 positioned on the pin and engaging the lever and case to urge the lever in the counterclockwise direction as seen in FIG. 5. The pivot arm 61 is pivotally mounted on a pin 65 carried in the case. The adjustment plate 62 includes a tongue 66 which engages an opening 67 in the pivot arm 61. A screw 68 passes through an opening 69 in the pivot arm 61 and engages a threaded opening '70 in the adjustment plate 62 for controlling the angular relation between the pivot arm and adjustment plate.

The latch lever 60 includes an end 73 for engaging the bimetal strip 32 and a notch 74 for engaging a tab.

75 of the pivot arm 61, as best seen in FIG. 5. A bracket 76 carried on the adjustment plate 62 engages the end of the bimetal strip 48 as best seen' in FIG. 3. The push rod 38 is carried in a cup formed by tongues 77, 78 of the plate 62, as best seen in FIGS. 5 and 6. A notch 79 is provided in the free end of the bimetal strip 49, permitting the strip to be positioned around the narrow body and engage the head 80 of an adjustment screw carried in the case 27.

The contact pair 16 and the heater 17 are positioned in the compartment 43. One contact of the pair 16 is carried on a thermal responsive element in the form of a bimetal strip 83 mounted ona bracket 84 and the other contact is carried on another thermal responsive element in the form of a bimetal strip 85 mounted on a bracket 86. A,

screw 87 is threadedly carried in an arm of the bracket 86 for engaging the strip 85 to vary its position relative to the strip 83. Typically the heater 17 comprises a plurality of turns of resistance wire wound on an insulating sheet 88 around the strip 83.

The operation of the switch is as follows. With the H room at the desired temperature, the room thermostat switch isopen, theburner is off and the blower is off. The contact pair 16 is closed and the contact pair 18 is open. When the temperature of the room falls below a preset value, the room thermostat switch 13 closesand the burner valve solenoid 14 is energized. It is assumed for the initial consideration of the operation that the contact pair 16 remains closed throughout the operation. When the room thermostat switch closes, the heater 15 is energized, applying heat to the bimetal strip 48. This heat causes the free end of the strip to move counterclockwise as viewed in FIGS. 5 and 6, which permits the pivot arm 61 and adjustment plate 62 to also move counterclockwise. This movement permits the push rod 38 to move to the right and utimately close the contact pair 18. The time delay between closing of the room thermostat switch 13 and the contact pair 18 is dependent upon the thermal characteristics of the bimetal strip 48 and the heater 15 and the dimensions of the various components. The delay may have any value but ordinarily is in the order of 45-75 seconds. Fine adjustments of the delay may be achieved by using the screw 68 and the screw 88. The blower relay 19 is energized when the contact pair 18 is closed, starting the blower, and the furnace continues to operate in this manner until the temperature of the room rises to a value which will cause the room thermostat switch to open.

Prior to energization of the burner valve and the heater 15, the latch mechanism is in the position shown in FIG. 6. As the temperature in the furnace increases, the free end of the bimetal strip 32 moves clockwise permitting the lever 60 to move counterclockwise. As the pivot arm 61 moves counterclockwise, the tab 75 engages the notch 74 of the latch lever, as seen in FIG. 5.

When the room thermostat switch opens, the burner valve and the heater 15 are de-energized. The bimetal element 48 cools and the free end thereof tends to move clockwise, applying a clockwise torque to the pivot arm 61 and adjustment plate 62. However, engagement of the tab 75 of the pivot arm with the notch of the latch lever blocks movement of the pivot arm and adjustment plate,

so that the contact pair 18 remains closed even though the heater 15 is no longer energized. Hence the blower remains on even though the burner hasvbeen shut off.

This condition will'obtain until the air temperature in the furnace falls, causing the free end of the bimetal strip 32 to move counterclockwise. counterclockwise movement of the bimetal strip 32 produces clockwise movement of the latch lever 60, releasing the pivot arm 61 to move the push rod to the left and open the contact pair 18. The relative movements of the elements of the mechanism during this operation are shown by the arrows in FIG. 6. When the contact pair 18 is open, the blower relay is deenergized and the blower is shut off.

Now the air temperature in the furnace will increase, again producing clockwise movement of the free end. of the bimetal strip 32. However, the blower will not again be energized since the heater 15 is not energized and the contact pair 18 is maintained open. There is no possibility of short cycling withthis blower control switch, as the blower can be energized only when the burner is also energized.

The bimetal strip 49 preferably is identical to the bimetal strip 48 and is included to provide compensation for changes in ambient temperature so that the blower start delay is independent of the air temperature in the vicinity of the furnace. The bimetal strip 49 may be omitted without affecting the operation of the switch as described above.

The heater 17 and the contact pair 16 provide for compensationof the blower start delay for variations in line voltage. The heat produced by the heater 15 and, hence, the time delay between burner start and blower start is a function of the voltage applied to the heater. For example, with the secondary voltage of the transformer nominally twenty-four volts, the actual voltage may vary over the range of twenty to thirty volts. The contact pair 16 and associated components are designed so that the contacts remain closed for a particular voltage at the secondary of the transformer 10 and will open when the voltage exceeds this particular value. For example, in the embodiment described herein, the heater 17 and the bimetal strip 83 are designed so that with twenty volts at the transformer secondary, the bimetal will remain stationary and the contact pair will remain closed. Then with continuous energization of the heater 15, the desired delay in blower start will be achieved. When the transformer secondary voltage is at a higher value, the free end of the bimetal element 83 will be moved counterclockwise, as seen in FIG. 3, opening the series circuit through the two heaters. Then the bimetal strip 83 will cool, again closing the contact pair 16 and again energizing both heaters. This cyclical operation will continue with the heater being energized when the contact pair 16 is closed so that sulficient heat is ultimately generated at the bimetal strip 43 to close the contact pair 18 and start the blower. The time required to open the contact pair 16 is an inverse function of the voltage, permitting the blower start delay to remain constant and independent of line voltage. The bimetal strip 85 is selected to have the same thermal response characteristic as the bimetal strip 83 to provide compensation for changes in ambient temperature. The bimetal strip 85 can be omitted where the surrounding temperature is relatively constant or where the effects of ambient temperature change can be ignored.

It is seen that the objects of the invention are achieved by the switch described above. Since the blower can be energized only as a function of heat produced by the heater 15, which in turn is energized through the room thermostat, operation of the blower at undesired times, such as the short cycling phenomenon, is eliminated. Also, since the blower is energized at a particular time following closing of the room thermostat switch and independently of the furnace air temperature, the construction and installation of the furnace blower switch is simplified. Particularly, it should be noted that the switch may be located on the furnace at a position where the bimetal strip 32 best senses the discharge air temperature for determining blower shutoff and that no concern need be given to the effect of furnace heat on the bimetal strip after blower shutdown.

Although an exemplary embodiment of the invention has been disclosed and discussed, it will be understood that other applications of the invention are possible and that the embodiment disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

I claim as my invention:

1. In a furnace blower switch, the combination of:

a housing adapted for mounting on a furnace wall, said housing including a bracket projecting into the furnace;

a snap action switch mechanism carried in said housing and movable between circuit open and closed conditions, said mechanism including means for biasing the switch toward the circuit closed condition;

a first thermal responsive element mounted in said housing and movable between normal and actuated positions;

meansfor coupling said first element to said switch mechanism for moving the switch to the open condition when said first element is in the normal position;

a latch mounted in said housing and movable between latched and unlatched positions;

means for urging said latch toward the latched position to engage and retain said first element when said first element is in the actuated position;

a heat source positioned at said first element for moving said first element to the actuated position;

means for energizing said heat source when the furnace blower is energized;

and a second thermal responsive element mounted in said bracket and movable between normal and actuated positions, With said second element disposed to engage said latch when moving to the normal position to move said latch to the unlatched position and release said first element, permitting the switch to move to the open condition.

2. In a furnace blower switch, the combination of:

a housing adapted for mounting on a furnace wall,

said housing including a bracket projecting into the furnace;

a snap action switch mechanism carried in said housing and movable between circuit open and closed conditions, said mechanism including means for biasing the switch toward the circuit closed condition;

a first thermal responsive element mounted in said housing and movable between normal and actuated positions;

means for coupling said first element to said switch mechanism for moving the switch to the open condition whensaid first element is in the normal position;

a latch mounted in said housing and movable between latched and unlatched positions;

means for urging said latch toward the latched position to engage and retain said first element when said first element is in the actuated position;-

a heat source positioned at said first element for moving said first element to the actuated position;

means for energizing said source when the furnace blower is energized, said means including a pair of contacts in series with said source and voltage sensitive means for oscillating said contacts between open and closed positions at a rate which is a function of the voltage applied thereto;

and a second thermal responsive element mounted in said bracket and movable between normal and actuated positions, with said second element disposed to engage said latch when moving to the normal position to move said latch to the unlatched position 7 and release said first element, permitting the switch to move to the open condition.

3. In a furnace blower switch, the combination of:

a housing adapted for mounting on a furnace wall, said housing including a bracket projecting into the furnace;

a snap action switch mechanism carried in said housing and movable between circuit open and closed conditions, said mechanism including means for biasing the switch toward the circuit closed condition;

a first thermal responsive element mounted in said housing and movable between normal and actuated positions;

means for coupling said first element to said switch mechanism for moving the switch to the open condi tion when said first element is in the normal position;

a latch mounted in said housing and movable between latched and unlatched positions;

' means for urging said latch toward the latched position to engage and retain said first element when said first element is in the actuated position;

a first heat source positioned at said first element for moving said first element to the actuated position;

means for energizing said first source when the furnace blower is energized, said means including a voltage compensation switch and a second heat source con nected in series with said first source, with said compensation switch having a second thermal responsive element for opening and closing the compensation switch, and with said second source positioned at said second element for opening the compensation switch when energized above a predetermined potential;

and a third thermal responsive element mounted in said bracket and movable between normal and actuated positions, with said third element disposed to engage said latch when moving to the normal position to move said latch to the unlatched position and release said first element, permitting the snap action switch to move to the open condition.

4. In a furnace blower switch, the combination of:

a housing adapted for mounting on a furnace wall, said housing including a bracket projecting into the fura snap action switch mechanism carried in said housing andmovable between circuit open and closed conditions, said mechanism including means for biasing the switch toward the circuit closed condition;

a first thermal responsive element pivotally mounted in said housing and movable between normal and actuated positions; 7

means for coupling said first element to said switch mechanism for moving the switch to the open condition when said first element is in the normal position;

I a latch mounted in said housing and movable between latched and unlatched positions;

' means for urging said latch toward the latched position to engage and retain said first element when said first element is in the actuated position;

a first heat source positioned at said first element for moving said first element to the actuated position;

means for energizing said first source when the furnace blower is energized, said means including a voltage compensation switch and a second heat source connected in series with said first source, with said compensation switch having a second thermal responsive element in one arm for opening and closing the compensation switch, and with said second source positioned at said second element for opening the compensation switch when energized above a predetermined potential;

a third thermal responsive element mounted in said bracket and movable between normal and actuated positions, with said third element disposed to engage said latch when moving to the normal position to move said latch to the unlatched position and release said first element, permitting the snap action switch to move to the open condition;

a fourth thermal responsive element mounted with said first element and engaging said housing for pivoting said first element relative to said housing as a function of ambient temperature to compensate said first element for changes in ambient temperature;

and a fifth thermal responsive element forming the other arm of said voltage compensation switch, with said second and fifth elements moving in the same direction for changes in ambient temperature.

5. In a furnace blower switch, the combination of:

a housing adapted for mounting on a furnace wall;

a switch mechanism carried in said housing and movable between circuit open and closed conditions;

a first thermal responsive element mounted in said housing and movable between normal and actuated positions;

means for coupling said first element to said switch mechanism for actuating the switch;

a latch mounted in said housing and movable between latched and unlatched positions;

means for urging said latch toward the latched position to engage and retain said first element when said first element is in the actuated position;

a heat source positioned at said first element for moving said first element to the actuated position;

means for energizing said heat source when the furnace blower is energized;

and a second thermal responsive element carried on said housing and projecting into the furnace and movable between normal and actuated positions, with said second element disposed to engage said latch when moving to the normal position to move said latch to the unlatched position, release said first element and actuate the switch to the open condition.

6. In a furnace blower switch, the combination of:

a housing adapted for mounting on a furnace wall;

a blower switch mechanism carried in said housing and movable between circuit open and closed conditions;

at first thermal responsive element mounted in said housing and movable between normal and actuated positions;

means for coupling said first element to said blower switch mechanism for actuating the blower switch;

a latch mounted in said housing and movable between latched and unlatched positions;

means for urging said latch toward the latched position to engage and retain said first element when said first element is in the actuated position;

a first heat source positioned at said first element for moving said first element to the actuated position; means for energizing said first source when the furnace blowertis energized, said means including a voltage compensation switch and a second heat source connected in series with said first source, with said compensation switch having a second thermal responsive element for opening and closing the compensation switch, and with said second source positioned at said second element for opening the compensation switch when energized above a predetermined potenand a third thermal responsive element carried on said housing and projecting into the furnace and movable between normal and actuated positions, with said third element disposed to engage said latch when moving to the normal position to move said latch to the unlatched position, release said first element and actuate the blower switch to the open condition. V

7. In a furnace blower switch, the combination of z a housing adapted for mounting on a furnace wall;

a blower switch mechanism carried in said housing and movable between circuit open and closed conditions; a first thermal responsive element mounted in said housing and movable between normal and actuated posiergization of the burner, said first means including electrical circuit means energized from the voltage source and responsive to energization of the furnace burner for powering said first means;

tions; voltage compensation means coupled between elecmeans for coupling said first element to said blower trical circuit means of said first means and the voltswitch mechanism for actuating the blower switch; age source for controlling power flow to said eleca latch mounted in said housing and movable between trical circuit means as a function of the potential latched and unlatched positions; of the source to maintain said predetermined time means for urging said latch toward the latched posi- 10 substantially constant;

tion to engage and retain said first element in one of a latch mechanism mounted in said housing for latchsaid positions; ing said blower switch in the on condition indea first heat source positioned at said first element for pendently of said first means;

moving said first element to said one position; and second means responsive to the air temperature means for energizing said first source when the furnace in the furnace for engaging said latch mechanism to blower is energized, said means including a voltage unlatch said blower switch without regard to the concompensation switch and a second heat source con dition of said first means when the furnace air temnected in series with said first source, with said comperature falls below a predetermined value. pensation switch having a second thermal respon- 10. In a furnace blower switch, the combination of: sive element for opening and closing the compensa- 0 a housing adapted for mounting on a furnace wall; tion switch, and with said second source positioned a snap action switch mechanism carried in said housing at said second element for opening the compensation and movable between circuit open and closed condiswitch when energized above a predetermined potentions, said mechanism including means for biasing tial; the switch toward the circuit closed condition; and a third thermal responsive element mounted in said a first thermal responsive element mounted in said housbracket and movable between normal and actuated g and movable between normal and heated P positions, with said third element disposed to engage tiOrlS; said latch when moving to the normal position to means for coupling said first element to said switch move said latch to the unlatched position and release m hanism for moving the switch to the open conaid fir t element and tuat the blow r wit h. dition when said first element is in the norm-a1 posi- 8. In a furnace blower switch for actuating a furnace tion; blower in conjunction with a circuit for energizing a a latch mounted in said housing and movable between furnace burner, the combination of: latched and unlatched Positions;

a housing adapted for mounting on a furnace wall; means for continuously urging said latch toward the a blower switch mechanism carried in said housing and latched Position to engage and retain Said first 6- movable b twe n blow r on and ff o ditio ment when said first element is in the heated posifirst means for moving said blower switch to the blower tion;

on condition following energization of the burner, a heat source positioned at said first element f0! IIIOV- said first means including electrical circuit means g Said first element tothe heated Position; responsive to energization of the furnace burner for m an for energizing Said heat Source When the fu ace powering said first means; blower is energized; and a lat h mechani m mounted in aid housing f l t ha second thermal responsive element carried on said ing said blower switch in the on condition indehousing and P j g into the furnace and r ponpendent of aid fir t m an sive to furnace temperature, said second element beand second means responsive to the air temperature in ing movable between normal and heated positions the furnace for engaging said latch mechanism to unand disposed to engage Said latch When moving to latch said blower switch without regard to the conthe normal Position to move Said l h t the undition of said first means when the furnace air temlatched Position and release Said first element, P perature falls below a predetermined valu mitting said first element to move the switch to the 9. In a furnace blower switch operable from a voltage p conditionsource for actuating a furnace blower in conjunction References Cited in fin file of this patent with a circult for energizing a furnace burner, the combination f; UNITED STATES PATENTS a housing adapted for mounting on a furnace wall; 1,010,414 Cubitt Dec. 5, 1911 a blower switch mechanism carried in said housing 2,235,337 Shaw Mar. 18, 1941 and movable between blower on and off conditions; 2,785,253 Dillman et a1 Mar. 12, 1957 first means for moving said blower switch to the blower 2,891,128 Bolesky June 16, 1959 on condition at a predetermined time following en- ,908,786 Schleicher Oct. 13, 1959 

8. IN A FURNACE BLOWER SWITCH FOR ACTUATING A FURNACE BLOWER IN CONJUNCTION WITH A CIRCUIT FOR ENERGIZING A FURNACE BURNER, THE COMBINATION OF: A HOUSING ADAPTED FOR MOUNTING ON A FURNACE WALL; A BLOWER SWITCH MECHANISM CARRIED IN SAID HOUSING AND MOVABLE BETWEEN BLOWER ON AND OFF CONDITIONS; FIRST MEANS FOR MOVING SAID BLOWER SWITCH TO THE BLOWER ON CONDITION FOLLOWING ENERGIZATION OF THE BURNER, SAID FIRST MEANS INCLUDING ELECTRICAL CIRCUIT MEANS RESPONSIVE TO ENERGIZATION OF THE FURNACE BURNER FOR POWERING SAID FIRST MEANS; A LATCH MECHANISM MOUNTED IN SAID HOUSING FOR LATCHING SAID BLOWER SWITCH IN THE ON CONDITION INDEPENDENT OF SAID FIRST MEANS; AND SECOND MEANS RESPONSIVE TO THE AIR TEMPERATURE IN THE FURNACE FOR ENGAGING SAID LATCH MECHANISM TO UNLATCH SAID BLOWER SWITCH WITHOUT REGARD TO THE CONDITION OF SAID FIRST MEANS WHEN THE FURNACE AIR TEMPERATURE FALLS BELOW A PREDETERMINED VALUE. 